How jack extension reduces a scaffold leg's allowable load — and what to do about it. Capacity tables for base jacks, U-heads, and combined extension, plus base pad bearing requirements.
A scaffold leg's allowable axial load is limited by buckling — not by the steel's yield strength. When you extend a base jack or U-head, you increase the unbraced length of the column, which sharply reduces the load it can carry before it buckles.
The effect is non-linear: doubling extension roughly halves capacity. Engineers ignoring this is one of the most common reasons that scaffolds fail under load on UAE sites.
Rule of thumb. Keep total jack extension (base + top) under 300 mm wherever possible. Above 300 mm, the column starts losing capacity faster than most BOQs account for. Above 600 mm extension on either end, get a competent designer to check the leg.
Capacity vs extension length, single 48.3 × 3.2 mm EN10219 standard with adjustable base jack, no eccentricity, properly tied scaffold. Safety factor 1.65 on Euler buckling.
| Base jack extension | Allowable axial load | % of max capacity |
|---|---|---|
| 0 mm (fully retracted) | 45.0 kN | 100% |
| 100 mm | 42.0 kN | 93% |
| 200 mm | 37.0 kN | 82% |
| 300 mm | 31.0 kN | 69% |
| 400 mm | 25.0 kN | 56% |
| 500 mm | 20.0 kN | 44% |
| 600 mm | 15.5 kN | 34% |
U-heads sit on top of the standard and carry the primary beam. Their extension stacks with the base jack extension to determine the total unbraced length at the top of the column.
| U-head extension | Allowable axial load | % of max capacity |
|---|---|---|
| 0 mm (fully retracted) | 45.0 kN | 100% |
| 100 mm | 42.0 kN | 93% |
| 200 mm | 37.0 kN | 82% |
| 300 mm | 31.0 kN | 69% |
| 400 mm | 25.0 kN | 56% |
| 500 mm | 20.0 kN | 44% |
| 600 mm | 15.5 kN | 34% |
When both jacks are extended, the unbraced column length is the sum. Use this combined table when planning slab support where both jacks need adjustment.
| Total extension (base + U-head) | Allowable axial load |
|---|---|
| 0 mm | 45.0 kN |
| 200 mm (100 + 100) | 39.0 kN |
| 400 mm (200 + 200) | 32.0 kN |
| 600 mm (300 + 300) | 25.0 kN |
| 800 mm (400 + 400) | 19.0 kN |
| 1000 mm (500 + 500) | 14.5 kN |
| 1200 mm (600 + 600) | 11.0 kN |
The other half of base jack design is what the jack sits on. The jack's base plate must spread the axial load onto a surface that won't punch through or settle. Typical bearing requirements:
| Surface | Capacity at base plate | Notes |
|---|---|---|
| Hardened RC slab | ≥ 50 kN per leg | Direct bearing acceptable |
| Compacted sub-base | 8–20 kN per leg | Use timber sole plate spreader (200 × 200 mm typical) |
| Granular / sand | ≤ 5 kN per leg | Always use a sole plate; consider geotextile or hardcore base |
| Newly cast slab (<7 days) | Reduced | Check with structural engineer; reshoring rules apply |
Common mistake. Putting full-capacity base jacks (45 kN rated) onto a compacted sand pad without a spreader plate. The jack is fine; the ground isn't. Punching settlement under one leg will redistribute load to neighbouring legs and cascade.
See also: Cuplock system specifications · Formwork beam load tables · Blog: Jack extension & load capacity
Our engineering team prepares stamped drawings and load checks for UAE projects.